Sub-miniature optical fiber cables, and apparatuses and methods for making the sub-miniature optical fiber cables

ABSTRACT

A simplex optical fiber cable of this invention includes an optical fiber, a buffer preferably of nylon, surrounding and in contact with the optical fiber, a yarn layer with strength fibers, preferably aramid fibers, disposed about the buffer and a sheath preferably formed of polyvinyl chloride (PVC) surrounding and in contact with the yarn layer. In cross-section, the simplex optical fiber cable has a diameter less than 2.0 millimeters (mm), and thus is much smaller in diameter than optical fiber cables presently available. Preferably, if the buffer is relatively tin providing limited protection to the optical fiber, a slick substance such as talc is applied to an outer surface of the buffer before the yam layer is disposed thereon. The slick substance allows the buffer of the optical fiber to slide to a degree in contact with the yarn layer and thus reduces fatigue caused by axial movement of a ferrule of the connector terminating the optical fiber cable. On the other hand, if the buffer is relatively thick, a friction-reducing substance such as Modaflo™ can be applied to the optical fiber to allow the buffer to be stripped relatively easily. A zip-cord duplex optical fiber cable of this invention includes essentially two simplex optical fiber cables with their respective sheaths joining at a middle portion along the axial length of the simplex optical fiber cables. Thus, in cross-section, the zip-cord duplex optical fiber cable has a figure-eight shape with a relatively thin portion in the middle which can be manually pulled apart to separate the zip-cord duplex optical fiber cable into separate simplex optical fiber cables. This feature of the invention allows the zip-cord duplex optical fiber cable to be split at its ends to allow connectors attached to respective ends of the optical fiber cables for connection to respective spaced connector receptacles. A second duplex optical fiber cable of this invention includes two simplex optical fiber cables arranged side-by-side with an oversheath extruded about and holding together the two simplex optical fiber cables. In cross-section, the two duplex optical fiber cables of this invention are less than 2.0 mm in height and 4.0 mm in width, and thus are much smaller than currently available duplex optical fiber cables. The invention also includes die assemblies and methods for making the simplex and duplex optical fiber cables.

RELATED APPLICATIONS

This application is a divisional application claiming priority toapplication Ser. No. 08/510,021; filed Aug. 1, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to a sub-miniature optical fiber cable, andto apparatuses and methods for making the sub-miniature optical fibercable.

2. Description of the Related Art

Local exchange carriers are increasingly using optical fiber signaltransmission in central offices to accommodate the increasing demand foroptical fiber systems such as fiber-to-the-home, fiber-to-the-curb,hybrid fiber-coax, digital loop carrier and interoffice carrier systems.The central offices are used to distribute optical fiber cables and toestablish cross-connections between optical fiber systems and/orexchanges. At present, optical fiber cable is produced in standard sizesof 2.4 mm or 3 mm in diameter. Although these standard sizes may appearto be relatively small in diameter, because they are used in such largenumbers in a central office, these standard sizes lead to significantcongestion, complication and expense in a central office. In fact, toaccommodate cross-connections between optical fiber systems orexchanges, central offices require a relatively large number of cabinetswith troughs housing optical fiber jumper cables, and racks housingconnectors to join optical fiber jumper cables together. The relativelylarge number of cabinets currently required in a central office toaccommodate cross-connections for optical fibers, increases the size andspace requirements for central offices and thus the expense of thecentral offices. Moreover, the relatively large standard sizes ofoptical fiber cables lead to congestion and complication in the centraloffice which requires significant time, and therefore expense, forservice persons to establish, replace, change or maintaincross-connections in the central offices. Further, the size of theoptical fiber cable has a multiplicative effect on the size of thecomponents that are used with the cable in the central office. Thus, ifthe optical fiber cable is relatively large, so must be the connectorswhich terminate and attach the optical fiber cable to other opticalfiber cables, the size of the racks that house the connectorreceptacles, the troughs which house the optical fiber jumper cables,and the size of the cabinets used to house the racks and troughs. If thesize of the optical fiber cables can be reduced, the connectors, racks,troughs and cabinets can be proportionally decreased in size. Alsonoteworthy is that the cost of the optical fiber cables, connectors andcabinets is proportional to the amount of materials used in themanufacture thereof. Therefore, by decreasing the size of the opticalfiber cables, connectors and cables, significant cost-savings can beobtained. Thus, there is a need to reduce the size of optical fibercables.

SUMMARY OF THE INVENTION

This invention overcomes the disadvantages noted above. In accordancewith this invention, a simplex optical fiber cable includes a jacketedoptical fiber at its core. The optical fiber is surrounded and contactedwith a buffer made of plastic material such as nylon, polyesters orpolyvinyl chloride (PVC). About the outer circumference of the bufferaramid yarn is disposed. A sheath of plastic material such as PVCsurrounds and contacts the aramid yarn.

A duplex optical fiber cable in accordance with this invention includestwo optical fibers with respective buffers and aramid yarn layers, whichare positioned side-by-side in a sheath, preferably of PVC, covering andcontacting the aramid yarn of both optical fibers and integratedtogether to form a continuous connection between portions of the sheathcovering respective aramid yarn layers, buffers and optical fibers. Inan alternative embodiment, a duplex optical fiber cable in accordancewith this invention includes two simplex optical fiber cables aspreviously described, positioned side-by-side and having an oversheath,preferably of PVC, enclosing the two simplex optical fiber cables.

In cross-section, the diameter of the simplex optical fiber cable canrange from 1.0 mm through 1.8 mm, but is preferably formed in standardsizes of about 1.2 mm and 1.6 mm. The simplex optical fiber cable ofthis invention is thus significantly smaller in diameter than theoptical fiber cables presently available. Likewise, in cross-section,the duplex optical fiber cable of this invention ranges from 2.76 mmthrough 4.25 mm in width and from 1.60 mm through 2.10 mm in height, butpreferably is formed in standard sizes of about 2.76 mm in width and1.68 mm in height, or about 4.20 mm in width and 1.60 mm in height.Thus, the duplex optical fiber cable of this invention is much smallerin cross-section compared to duplex optical fiber cables presentlyavailable.

Thus, the simplex and duplex optical fiber cables of this invention areminiaturized relative to prior art optical fiber cables, and makepossible the reduction of congestion, complication and size and spacerequirements presently required in central offices. These features ofthe simplex and duplex optical fiber cables of this inventionconsiderably reduce the costs of establishing, operating and maintainingcentral offices.

In accordance with this invention, if the buffer surrounding the opticalfiber is relatively thin in either the simplex or duplex cableconfiguration, a slick substance such as talc can be applied to theouter surface of the buffer. This feature of this invention allows forthe buffer to slide in contact with its aramid yam layer so that theoptical fiber will not be overbent when the ferrule of a connectorattached to an end of the optical fiber, forces the optical fiber in anaxial direction along the length of the optical fiber cable as theconnector is connected to a connector receptacle. Thus, the simplex andduplex optical fiber cables of this invention reduce damage or breakageof optical fibers caused by overbending or fatigue when a connector iscoupled to a connector receptacle.

Also, in either the simplex or duplex cable configurations, if thebuffer is relatively thick, a slick substance such as Modaflo™ can beapplied to the outer surface of a coated optical fiber 4 so that therelatively thick buffer can more readily be stripped from the opticalfiber.

In addition, the duplex optical fiber cables of this invention areadvantageous in that the respective buffers of the two optical fibers ineach duplex optical fiber cable of this invention are individuallywrapped with aramid yam as opposed to wrapping aramid yarn around thebuffers of both optical fibers as done in one type of optical fibercable presently available. By individually wrapping the two opticalfiber buffers in the duplex optical fiber cable of this invention, theneed for bifurcation kits to connect the duplex optical fiber cable tosingle connectors, is eliminated. Because bifurcation kits have partswhich force the diameter of the optical fiber cable to increase greatly,the elimination of the need for bifurcation kits effectively reduces thesize of the duplex optical fiber cable of this invention relative topresently available duplex optical fiber cables. Also, bifurcation kitsare relatively expensive, so the elimination of the need to usebifurcation kits in the duplex optical fiber cables of this inventionprovides significant cost-savings relative to duplex optical fibercables which require bifurcation kits.

These together with other objects and advantages, which will becomesubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings, forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be better understood with reference to thefollowing drawings. The drawings are not necessarily to scale, emphasisinstead being placed upon clearly illustrating principles of the presentinvention.

FIG. 1 is a diagram of an optical fiber jumper cable in accordance withhis invention with connectors at the terminal ends thereof, showing theoperation of connecting the connectors to connector receptacles coupledto respective optical fiber cables;

FIG. 2 is a perspective cutaway view of a simplex optical fiber cable inaccordance with this invention;

FIG. 3 is a cross-sectional view of the simplex optical fiber cable;

FIG. 4 is an exploded perspective view of a crosshead assembly formaking the simplex optical fiber cable;

FIG. 5 is a cross-sectional diagram of the crosshead assembly for makingthe simplex optical fiber cables, shown in its assembled configuration;

FIG. 6 is a cross-sectional view of a duplex optical fiber cable inaccordance with this invention;

FIG. 7 is an exploded perspective view of a crosshead assembly formaking the duplex optical fiber cable of FIG. 6;

FIG. 8 is a cross-sectional view of the crosshead assembly of FIG. 7,shown in its assembled configuration; and

FIG. 9 is a cross-sectional view of a second embodiment of the duplexoptical fiber cable in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, an optical fiber cable 1 has connectors 2 attached to theterminal ends thereof The connectors 2 can be ST, FC, or SC connectors,for example. The connectors 2 can be connected by insertion intorespective connector receptacles 3. The connector receptacles 3 areconnected to respective optical fiber cables I for distribution toremote locations.

In a central office, connector receptacles 3 are typically housed inracks (not shown) in cabinets (not shown). Also, the central officehouses the optical fiber cables 1 in troughs (not shown) in and runningbetween the cabinets. Because a relatively large number ofcross-connections are required in a central office to establishconnections to transmit optical signals from various remote locations toothers, a correspondingly large number of connector receptacles 3 andoptical fiber cables 1 are housed in the central office. The opticalfiber cable 1 of this invention is relatively small in diameter comparedto currently-used optical fiber cables, so the use of the optical fibercables 1 of this invention greatly reduce confusion, congestion, andcomplication in establishing, organizing, replacing or maintainingcross-connections in the central office. In addition, the relativelyreduced size of the optical fiber cable 1 of this invention allows for areduction in the size of the connectors 2 and the connector receptacles3, a feature which allows for the reduction of cabinet sizes relative tocurrently-used cabinets. In turn, the reduction of the size of thecabinets used in the central office leads to a reduction in the amountof floor space required for the cabinets. Because the amount of floorspace in a central office determines the expense of building, leasing,and/or operating a central office, this reduction in the cabinet sizesused in a central office leads to a significant cost savings for thecentral office.

In FIG. 2, a simplex optical fiber cable 1 of this invention is shown ina perspective cutaway view. The simplex optical fiber cable 1 includesat its core an optical fiber 4. Preferably, the optical fiber 4 iscoated with a plastic material such as ultraviolet (UV)-curableacrylate, to provide a degree of protection for the optical fiber 4.Typically, the coated optical fiber 4 has a diameter of 0.254 mm(0.010"). Surrounding the optical fiber 4 and in contact with the outersurface thereof, a buffer 5 is formed. The buffer 5 is preferably formedof a plastic material such as nylon, although other types of plasticmaterial can be used to form the buffer 5. Nylon material is preferredfor the buffer 5, however, because it has a degree of stiffness which isrelatively high for plastic materials. Therefore, the buffer 5 composedof nylon can be formed with a diameter which is relatively thin, and yetthe nylon buffer 5 is not inhibited by its relatively thin diameter fromproviding significant protection from overbending of the optical fiber4. Thus, the use of nylon to form the buffer 5 allows forminiaturization of the size of the optical fiber cable relative to otheroptical fiber cables.

If the buffer 5 is relatively thick (900 mm in diameter, for example), acoating of Modaflo™ (a mixture of Teflon® and acetone is applied to thecoated optical fiber 4 before forming the buffer 5 thereon. Because thehoop stress of the buffer 5 upon the optical fiber 4 is relatively highif the buffer 5 is relatively thick, the use of the Modaflo™ coatinghelps to reduce friction between the buffer 5 and they optical fiber 4so that the buffer 5 can readily be stripped off of the optical fiber 4.For example, the Modaflo™ coating helps to strip off the relativelythick buffer 5 when attaching a connector to the simplex optical fibercable 1.

On the other hand, if the buffer 5 is relatively thin (e.g., 500 micronsin diameter) a relatively slick substance 6 is applied about the outersurface of the buffer 5. The substance 6 can be talc, for example. Thesubstance 6 facilitates sliding of the buffer 5 relative to a yam layerso that the buffer 5, and, therefore the optical fiber 4, will not beoverbent when the buffer 5 is forced to slide relative to the yarn layer7, for example, when connecting a connector attached to the end of theoptical fiber cable 1 to a connector receptacle. The substance 6 is notnecessary if the buffer 5 is relatively thick, because in this case thebuffer 5 is sufficiently strong to prevent the optical fiber 4 frombeing overbent. About the outer surface of the buffer 5, the yarn layer7 is composed of strands either laid straight (i.e., parallel with theoptical fiber 4) or helically wrapped. Preferably, the yam layer 7includes yam strands with aramid strength fibers which have relativelyhigh strength and resistance to stress and strain. The yarn layer 7prevents the optical fiber 4 from being damaged by overbending. Also,because the yam layer 7 is composed of relatively strong aramid fibers,the yarn layer 7 is subject to relatively little fatigue over time. Inaddition, the yarn layer 7 provides significant protection for thebuffer 5 and the optical fiber 4 from impact or shock with an object, orfrom inadvertent cutting or tearing of the optical fiber cable 1. Aboutthe outside surface and in contact with the outer surface of the yarnlayer 7, a sheath 8 is formed. The sheath 8 is formed from a plasticmaterial such as polyvinyl chloride (PVC) (or more generally, plenum,riser and non-halogen rated plastics). The sheath 8 provides structuralstrength for the optical fiber cable 1 and is flexible to a degree, butalso is sufficiently resilient to prevent the optical fiber 4 from beingdamaged by overbending.

In diameter, the simplex optical fiber cable of FIG. 2 ranges from 1.0mm to 1.8 mm in diameter, and thus is much smaller than the standardsizes of 2.4 mm or 3 mm in diameter for optical fiber cables that aretypically available. More specifically, the optical fiber 4 has adiameter of 250 microns (±15 microns) in diameter, the buffer 5 rangesfrom 0.1 mm to 0.31 mm in radial thickness, the yarn layer 7 ranges from0.22 mm to 0.52 mm in radial thickness, and the sheath 8 ranges inthickness from 0.15 mm to 0.25 mm. Preferably, 15 the simplex opticalfiber cable 1 of this invention has standard sizes of about 1.2 mm and1.6 mm in diameter. For the first standard size of about 1.2 mm, thecoated optical fiber 4 is about 0.250 mm in diameter, the buffer 5 isabout 0.13 mm in radial thickness, the yarn layer 7 is about 0.22 mm inradial thickness, and the sheath 8 is about 1.8 mm in diameter. For thesecond standard size of the simplex optical fiber cable 1 of thisinvention, the coated optical fiber 4 is about 0.25 mm in radialthickness, the buffer 5 is about 0.32 mm in diameter, the yarn layer 7is about 0.22 mm in radial thickness, and the sheath 8 is about 0.1 mmin radial thickness.

In FIG. 3, the simplex optical fiber cable 1 of FIG. 2 is shown incross-section. The elements of FIG. 3 were previously described withrespect to FIG. 2, but the cross-sectional view of FIG. 3 is provided togive an understanding of the simplex optical fiber cable 1 of thisinvention in three-dimensions.

Importantly, if the nylon buffer 5 is formed with a diameter of about500 microns, the inventors have found that the buffer 5 can be strippedwith a force of 3 pounds or less, a feature which greatly eases theoperation of attaching a connector to the optical fiber cable 1, forexample. The inventors have determined that a diameter of about 900microns for the buffer 5 is too great to strip the buffer 5 withoutapplying an intermediate layer of a substance such as Modaflo™, whichallows the 900 micron nylon buffer to be stripped from the optical fiber4. In any case, the maximum diameter of the nylon buffer 5 for which thebuffer 5 can be stripped with a force of 3 pounds or less lies between900 and 500 microns.

In FIG. 4, a crosshead assembly 9 for making the simplex optical fibercable 1 of this invention is shown. The crosshead assembly 9 includes acrosshead 10 which can be a standard type of crosshead widely used inthe plastics extrusion industry. The crosshead 10 defines an open cavity11 and an aperture 12 at a first end of the crosshead 10 whichcommunicates with the cavity 11. Opposite its first end, the crossheaddefines a second, open end formed by the opening of the cavity 11. Atthe second end of the crosshead 10 about the cavity 11, are definedthreads 13 (not shown in FIG. 4, but shown in FIG. 5). The crosshead 10also defines on its side surface an aperture 14 to receive an alignmentpin (not shown) to achieve proper orientation of the parts of thecrosshead assembly 9. The crosshead 10 further defines an aperture 15 onits side surface, to allow insertion of a temperature probe (not shown)for monitoring the temperature of extruded plastic material. Thecrosshead 10 also includes a flat surface 16 which can be engaged with asupport (not shown) with screws (not shown) threaded through respectiveapertures 17 defined in flanges 18 of the crosshead 10.

The aperture 12 is circular in cross-section and sized to receive andhold, when inserted into the second open end and through the cavity 11defined in the crosshead 10, the cylindrical surface 19 situated at afirst end of a die holder 20. On a second end opposite its first end,the die holder 20 also has a cylindrical surface 21 with a diameterlarger than that of the cylindrical surface 19 and thus defining a ledge22 which, when inserted into the crosshead 10, engages with an innersurface of the first end of the crosshead 10 in proximity to theaperture 12 to fix the die holder 20 in position against the insidesurface of the first end of the crosshead 10. The die holder 20 alsodefines at its center an aperture 23 extending along an axial length ofthe die holder 20 so that the die holder 20 is effectively ring- ordisc-like in shape. The aperture 23 is defined in the die holder 20 sothat the die holder 20 has two cylindrical surfaces 24, 25 (only surface24 is visible in FIG. 4, but FIG. 5 shows both surfaces 24 and 25). Thecylindrical surface 25 has a greater diameter than the cylindricalsurface 24, thus defining a step 26 (not shown in FIG. 4, but shown inFIG. 5) therebetween. About an outer periphery on the second side of thedie holder 20, a recessed portion 27 is defined adjacent the cylindricalsurface 21. The purpose of the recessed portion 27 will be describedlater in this document. In addition, the die holder 20 has a recess 28for receiving an alignment pin (not shown) inserted through the aperture14 of the crosshead 10, to orient and lock the die holder 20 in thecrosshead 10.

The crosshead assembly 9 also includes a die 29. The die 29 defines anaperture 30 centered in and extending along the axial length of the die29. The aperture 30 is defined in the die 29 such that it has acylindrical surface 31 (not shown in FIG. 4, but shown in FIG. 5) inproximity to a first end of the die 29, and such that it has afunnel-like portion 32 (not shown in FIG. 4 but shown in FIG. 5) inproximity to a second end of the die 29. The funnel-like portion 32 isdefined so that it narrows from the second end toward the first end ofthe die 29 along an axial length thereof until meeting with thecylindrical surface 31. The cylindrical surface 31 of the die 29 shapesthe molten plastic material to form the outside surface of the sheath 8of the simplex optical fiber cable 1 of this invention, as will beexplained later in this document. The die 29 includes outer cylindricalsurfaces 33, 34. When the die holder 20 is assembled with the die 29,the surface 33 is inserted into the aperture 23 defined by the dieholder 20. The cylindrical surface 34 has a larger diameter than that ofthe cylindrical surface 33, and thus defines a ledge 35 which engageswith a second end of the die holder 20 when the die holder 20 isassembled together with the die 29. Also, when the die 29 is insertedinto the die holder 20, the second ends of the die holder 20 and the die29 are flush and define a substantially uniform flat surface. This flatsurface engages with a first end of a core tube 36 to enclose passages37, 38 and annular recess 39 defined on the first end face of the coretube 36. To hold the core tube 36 in position relative to the die holder20, the core tube 36 has a rim 40 disposed about an outer circumferenceof the core tube 36, which engages with the recessed portion 27 definedabout the outer circumference of, and on the second end of, the dieholder 20. The first end of the core tube 36 defines a notch 41 whichreceives molten plastic material such as nylon or PVC for the extrusionof the sheath 8 of the simplex optical fiber cable 1. When enclosed bythe flat surfaces of the second ends of the die 29 and the die holder20, the notch 41 together with a portion of the cylindrical surface 21of the die holder 20 define an aperture to receive the molten plasticmaterial. The passages 37, 38 are U-shaped in cross-section and togetherwith the respective flat surfaces of the second ends of the die holder20 and the die 29, define channels through which the molten plasticmaterial flows. The passages 37, 38 split the molten plastic materialflow from the notch 41 and channel the split flows of molten plasticmaterial to opposite sides of a circular aperture 42 defined at a centeraxis of the core tube 36. The passages 38 further split the flow fromrespective passages 38 and direct the flow of plastic material to theannular recess 39 at four spaced locations provided at 90 degree angularintervals about the edge of the circular aperture 42. The core tube 36also has a rim 43 which extends from the second end of the core tube 36.Defined symmetrically in the rims 40, 43 are opposing notches 44, 45(not all of which are shown). The notches 44, 45 allow insertion of thetip of a screwdriver, for example, to disassemble the die holder 20 andthe entry die 55 from the core tube 36.

A core tube insert 46 has a tip 47 with an aperture 48 formed therein.The aperture 48 is defined by the core tube insert 46 such that itextends along the axial length of the core tube insert 46. At a firstend of the core tube insert 46, the aperture 48 has a cylindricalportion 49 (not shown in FIG. 4, but shown in FIG. 5). At a second endof the core tube insert 46, the aperture 48 has a funnel-like portion 50(not shown in FIG. 4, but shown in FIG. 5) which converges in adirection from the first end to the second end of the core tube insert46 until meeting with an end of the cylindrical portion 49 inside of thecore tube insert 46. Adjacent the tip 47, the core tube insert 46 has anouter conical portion 51 about which molten plastic material is extrudedby the passages 37, 38 and the annular recess 39 of the core tube 36.Also, the core tube insert 46 includes outer cylindrical surfaces 52,53. The cylindrical surface 52 adjacent an end of conical portion 51,has a diameter smaller than that of the cylindrical surface 53 and thusdefines a ledge 54 between the cylindrical surface 52, 53. When the coretube insert 46 is assembled with core tube 36, the ledge 54 engages withthe second side of the core tube 36, to hold the core tube insert 46 inposition and prevent the core tube insert 46 from moving in a directiontoward the right in FIG. 4. Also, the cylindrical surface 52 is sized tofit snugly in the aperture 42 defined in the core tube 36 to hold thecore tube insert 46 firmly in position therein. When the core tubeinsert 46 is assembled together with the core tube 36 and the die 29,the conical portion 51 extends through the aperture 42 and the tip 47extends into the aperture 30 at the second side of the die 29.

An entry die 55 defines a funnel-like aperture 56. The funnel-likeaperture 56 converges or tapers from the second end of the entry die 55to its first end, and so is relatively open at the second end of theentry die 55, and relatively closed at the first end of the entry die55. The entry die 55 also has a recess 57 formed about the periphery ofthe entry die 55 on its first end, which receives the rim 43 of the coretube 36 to aid in holding the core tube 36, the core tube insert 46 andthe entry die 55 together when assembled. A crosshead nut 58 has at itsfirst end threads 59 and at its second end hexagonal surfaces 60. Thethreads 59 mate with corresponding threads 13 of the crosshead 10. Whenthreaded to the crosshead 10, the crosshead nut 58 holds the die holder20, the die 29, the core tube 36, the core tube insert 46, and the entrydie 55 in assembly inside of the cavity 11 of the crosshead 10. Thehexagonal surfaces 60 allow a wrench (not shown) or the like to be usedto screw the threads 59 of the crosshead nut 58 onto correspondingthreads 13 of the crosshead 10. The crosshead nut 58 defines an aperture61 extending along the axial length thereof. When assembled with theentry die 55, the aperture 61 communicates with the aperture 56 definedin the entry die 55.

The cross head 10, die holder 20, die 29, core tube 36, core tube insert46, the core guide 55 and the cross head nut 58 can all be made of metalmaterial such as stainless steel or tool steel.

In FIG. 5, the crosshead assembly 9 for making the simplex optical fibercable 1 in accordance with this invention, is shown in cross-section.The crosshead assembly 9 is assembled by inserting the first side of thedie 29 into the second side of the die holder 20. When so inserted, theouter cylindrical surface 33 of the die 29 meets with the innercylindrical surface 24 of the die holder 20, the ledge 35 abuts step 26and the outer cylindrical surface 34 contacts the inner cylindricalsurface 25. The step 26 and the ledge 35 fix the die 29 in position andprevent the die 29 from moving toward the right in FIG. 5. The core tube36 is joined with the die holder 20 so that the rim 40 meets withrecessed portion 27, thus holding the die 29 between the die holder 20and the core tube 36. The core tube insert 46 is inserted into thesecond side of the core tube 36, so that tip 47 of the core tube insert46 is inserted through the core tube 36 and into the die 29 so that thetip 47 is situated at the first side of the die 29 in the aperture 30.When so inserted, the conical portion 51 of the core tube 46 opposes thefunnel-like portion 32 and the cylindrical surfaces 52, 53 and ledge 54meet with respective surfaces defining the aperture 42 in the core tube36. The entry die 55 is joined with the core tube 36 so that its recess57 meets with the rim 43. The assembled die holder 20, die 29, core tube36, core tube insert 46 and entry die 55 are inserted into the cavity 11of the crosshead die 10 until the ledge 22 meets with an annular step 62defined in the crosshead 10. The assembled die holder 20, die 29, coretube 36, core tube insert 46 and entry die 55, are held in position inthe crosshead 10 by screwing the threads 59 of the crosshead nut 58 intothe threads 13 of the crosshead 10.

To make the optical fiber cable 1 of FIGS. 2 and 3, the optical fiber 4is manufactured and preferably coated using well-known techniques. Ifthe buffer 5 is to be relatively thick (900 microns in diameter, forexample), a coating of a friction-reducing substance such as Modaflo™ isapplied to the optical fiber 4, for example, by drawing the opticalfiber 4 through a container holding such substance. The buffer 5 is thenextruded onto the optical fiber 4 using well-known techniques. If thebuffer 5 is relatively thin (500 microns in diameter, for example), thecoating of the substance such as Modaflo™ can be omitted. On the otherhand, if the buffer 5 is relatively thick (i.e., 900 microns indiameter) the slick substance 6 is applied to the buffer 5 afterextrusion of the buffer 5, either by spraying or dusting the slicksubstance 6 on the buffer 5, or by running the optical fiber 4 through acontainer holding the slick substance 6. The straight-laying or helicalwrapping of the yarn layer 7 on the buffer 5 can be performed by anorganizer (not shown) situated to the left in FIG. 5 relative to thecrosshead assembly 9. Such organizers are well-known in the opticalfiber cable industry. The organizer can be a circular ring with holesformed therein to receive strands of the yarn 7. If the strands of yarnare to be laid straight (i.e., parallel with the optical fiber 4), thestrands of yarn are advanced through the organizer's holes and guidedinto contact with the buffer 5 advanced by a motor through the center ofthe organizer. Alternatively, if the yarn strands are to be helicallywrapped onto the buffer 5, the organizer ring is rotated by a motor (notshown) which causes the strands of yarn 7 to be helically wrapped aboutthe buffer 5 as it is drawn through the center aperture of the circularring. The optical fiber 4 with buffer 5 and aramid yarn layer 7, areinserted from the left side of FIG. 5 into apertures 61, 56, 48 andthrough apertures 23 and 12 of the crosshead assembly 9 of FIGS. 4 and5. The apertures 56 and 48 define a funnel shape which tends to guideand ease insertion of the end of the optical fiber 4, the buffer 5 withapplied substance 6, and wrapped aramid yarn 7, through the crossheadassembly 9. Thus, the funnel shape of the apertures 56, 48 greatly easesthe preparation of the crosshead assembly 9 for extrusion of the sheath8 relative to previously-used crosshead dies.

The coated optical fiber 4, buffer 5 (with applied substance 6, ifused), and yarn layer 7 are drawn through the apertures 61, 56, 48, 23and 12 with a motor (not shown). Molten plastic material is forced intothe crosshead assembly 9 and through the notch 41 into passages 37, 38which split the flow of molten plastic material from the notch 41 intosplit flows supplied at separated locations about the circumference ofthe annular recess 39 of the core tube 36. The annular recess 39 evenlydistributes the molten plastic material about the annular surface 39 ofthe core tube insert 46. The molten plastic material flows in a passagedefined by the outer surface of the conical portion 51 of the core tubeinsert 46 and the inner surface of the aperture 30 defined in the die29. Advantageously, the channel defined between the outer surface of theconical portions 51 of the core tube insert 36 and the inner surface ofthe funnel-like portion 32 of the die 29 cause the flow of moltenmaterial to converge toward the tip 47 of core tube insert 46, a featurewhich ensures that the flow of molten plastic material is uniformlydistributed and continuous about the circumference of the tip 47. Themolten plastic material flows over the outer surface of the tip 47 andthe inner surface of the cylindrical surface 31 defining the aperture 30at the first side of the die 29, forming a sheath 8, ring-like incross-section, about the optical fiber 4, the buffer 5, (with theapplied substance 6, if used), and the yarn layer 7 as they are drawnthrough the crosshead assembly 9. When cooled sufficiently, the sheath 8constricts to a degree on the yarn layer 7 to form the simplex opticalfiber cable 1 of this invention.

The molten plastic material is extruded at a temperature of about 360°F. and cooled by immersion in 40°-60° F. water.

Importantly, should the die 20, the core tube 36 and/or the core tubeinsert 46 become clogged with plastic material, they can be readilyreplaced individually without the expense of replacing the entirecrosshead assembly 9, a feature which provides significant cost-savingsrelative to previously-used dies.

FIG. 6 is a cross-sectional diagram of a first embodiment of a duplexoptical fiber cable 1 in accordance with this invention. Essentially,the first embodiment of the duplex optical fiber cable 1 includes twosimplex optical fiber cables 1 (as shown in FIGS. 2 and 3), but with therespective sheaths 8 of the two simplex optical fiber cables 1 beingformed such that they have a continuous connection between the twosimplex optical fiber cables 1. The first embodiment of the duplexoptical fiber cable 1 is called a zip-cord configuration and can bepulled apart at the middle connecting portion of the sheath 8 to allowseparation between the two duplex optical fiber cables 1 so thatrespective connectors can be attached to respective ends of each opticalfiber 4. This separation of the ends of the optical fiber cable 1 intotwo simplex optical fiber cables 1 allows the connector to be connectedto spaced connector receptacles. After splitting the end of the duplexoptical fiber cable 1 at the middle portion along a length sufficient toconnect the optical fibers 4 to respective spaced connector receptacles3, the duplex optical fiber 1 can be taped with an adhesive tape aboutits circumference so that the zip-cord duplex optical fiber cable 1 willnot further split at its middle portion. This feature of the inventioneliminates the need for bifurcation kits required to split the terminalend of previously-used miniature duplex optical fiber cables for theattachment of connectors thereto. Because a bifurcation kit hascomponents which greatly increase the diameter of a duplex optical fibercable to which the bifurcation kit is attached, the duplex optical fibercable 1 of this invention is substantially reduced in size compared topreviously-used optical fiber cables, a feature which leads to reducedcongestion in the racks and troughs of central office cabinets.

In cross-section, the zip-cord duplex optical fiber cable 1 of thisinvention can range in size from 2.20 mm through 4.25 mm in width andfrom 1.25 mm through 2.00 mm in height (with an optical fiber of 0.254mm in diameter, an optical fiber buffer of 0.1-0.34 mm in radialthickness, a yarn layer of 0.22-0.52 mm in radial thickness and a sheathof 0.15-0.25 mm in radial thickness but preferably formed in standardsizes of about 2.7 mm in width and 1.68 mm in height, or about 3.55 mmin width and 1.60 mm in height (for the first standard size, the opticalfiber is 0.254 mm in diameter, the buffer is 0.13 mm in radialthickness, the yarn layer is 0.22 mm in radial thickness and the sheathis 0.18 mm in radial thickness, and for the second standard size theoptical fiber is 0.254 mm in diameter, the buffer is 0.32 mm in radialthickness, the yarn layer is 0.22 mm in radial thickness and the sheathis 0.18 mm in radial thickness). Thus, the zip-cord duplex optical fibercable 1 of this invention is much smaller in cross-section compared toduplex optical fiber cables presently available.

FIG. 7 is an exploded perspective view of a crosshead assembly 9 formaking the zip-cord duplex optical fiber cable 1 of this invention. Thecrosshead 10 has similar components of those explained previously withrespect to FIG. 4, so an explanation of these elements will be omittedhere. The crosshead assembly 9 of FIG. 7 also includes a die 65 defininga figure-eight-shaped aperture 66 at a first end of the die 65. Theaperture 66 had a shape conforming to two tubes placed side-by-side suchthat the tubes intersect and have an open space at the intersectingportion thereof. The die 65 also defines outer cylindrical surfaces 67,68. The cylindrical surface 67 has a diameter less than that of thecylindrical surface 68 so that the cylindrical surfaces 67, 68 define aledge 69 therebetween. When inserted into the cavity 11 of the crossheaddie 10 of FIG. 7, the ledge 69 meets with the face of annular step 62(not shown in FIG. 7, but shown in FIG. 8) and thus prevents the die 65from moving toward the right in FIG. 7. The die 65 also has a rim 70extending from a second end of the die 65 from the outer periphery ofthe cylindrical surface 68. The die 65 also defines slots 71, 72. Theslot 71 receives an alignment pin inserted through the aperture 14 ofthe crosshead 10, to align and lock the position of the die 65 in thecrosshead 10. The slot 72 allows for the tip of the screwdriver or thelike to be inserted into the slot 72 to separate the die 65 from otherparts of the crosshead assembly 9. Centered at its second side andextending along the axial length thereof the die 65 defines a conicalsurface 73 (not shown in FIG. 7, but shown in FIG. 8) which convergesfrom the second side to the first side of the die 65. The conicalsurface 73 defines an aperture 74 which communicates with thefigure-eight-shaped aperture 66.

The crosshead assembly 9 of FIG. 7 also includes a core tube 75 defininga notch 76 at one side thereof. The notch 76 receives molten plasticmaterial from the aperture 64 of the crosshead 10, to extrude the sheath8 of the zip-cord duplex optical fiber cable 1 of this invention. On itsfirst end face, the core tube 75 defines passages 77 which split theflow of molten plastic material from notch 76 and guide the split flowsof molten plastic material to opposite sides of a conical portion 78disposed on the first end of the core tube 75 and extending along theaxial length thereof. The conical portion 78 has recessed surfaces 79which guide respective split flows along the conical portion 78. Throughthe center of the conical portion 78, an aperture 81 is defined whichruns from the tip end of the conical portion 78 along the axial lengthof the core tube 75. The aperture 81 is defined at the first end of thecore tube 75, by a surface 82 (not shown in FIG. 7, but shown in FIG. 8)which in cross-section has two parallel, opposing sides with respectiveopposing semicircular ends meeting with respective ends of the opposingsides. Communicating with the aperture 81 defined at the first end ofthe core tube 75, a conical surface 83 (not shown in FIG. 7, but shownin FIG. 8) is defined in proximity to the second end of the core tube75. Between the adjoining ends of the surface 82 and the conical surface83, a step 84 (not shown in FIG. 7, but shown in FIG. 8) is defined inthe core tube 75. The core tube 75 has a rim 85 extending from thesecond end thereof from the outer periphery of cylindrical side surface86 of the core tube 75. The core tube 75 also includes opposing notches87 to allow the crosshead assembly 9 to be disassembled using the tip ofa screwdriver, for example.

The crosshead assembly 9 for making the zip-cord duplex optical fibercable 1 of this invention also includes a core tube insert 88 includinga conical portion 89 with an extension 90 protruding from and formedintegrally therewith. In cross-section, the extension 90 has outersurfaces with two opposing, parallel sides and respective semicircularsurfaces at the respective ends of the opposing, parallel sides.Extending from the extension 90, two parallel tubes 91 are disposed. Thetubes 91 extend along the axial length of the core tube insert 88 fromits first end to a location in near proximity to the second end of thecore tube insert 88. The tubes 91 have inner surfaces definingrespective apertures 92. In proximity to the second end of the core tubeinsert 88, the core tube insert 88 defines conical surfaces 93communicating with the apertures 92 defined by the tubes 91. The conicalsurfaces 93 are relatively open at the second end of the core tubeinsert 88, but converge in a direction toward the first end of the coretube insert 88 until meeting with respective ends of the tubes 91.

The crosshead assembly 9 of FIG. 7 also includes a core guide 94 whichis substantially cylindrical in shape and includes a conical surface 95protruding at the center of its first end. The conical surface 95 has aflat end surface 96 defining a figure-eight-shaped aperture 97 having ashape conforming to two tubes with a spaced portion at the intersectionof the two tubes. The figure-eight-shaped aperture 97 extends along theaxial length of the core guide 94 and tapers inward from a second end tothe first end of the core guide 94 (as shown in FIG. 8). The core guide94 also has a recessed portion 98 about its outer periphery at the firstend thereof The crosshead assembly 9 of FIG. 7 also includes a crossheadnut 99 having threads 100 defined at its first end, and having hexagonalsurfaces 101 defined at its second end. The threads 100 can be threadedonto corresponding threads 13 defined at the second end of the crosshead10 by rotating the crosshead nut 99 relative to the crosshead 10. Thehexagonal surfaces 101 allow a wrench or the like to be fitted theretofor use in screwing the crosshead nut 99 into the mating threads 13 ofthe crosshead 10. The crosshead nut 99 also defines an aperture 102extending along the axial length thereof.

The cross head 19, die 65, core tube 75, core tube insert 88, core guide94 and cross head nut 99, can all be made of metal material such asstainless steel or tool steel.

FIG. 8 is a cross-sectional view of the crosshead assembly 9 in itsassembled state. The crosshead assembly 9 is assembled by joining thefirst end of the core tube 75 with the second end of the die 65 so thatthe conical portion 78 of the core tube 75 is inserted into and opposesthe conical surface 73 of the die 65, and so that the flat surface ofthe first end of the core tube 75 contacts the flat surface of thesecond end of the core die 65 to enclose the passages 78 and a side ofthe notch 76. As so fitted together, the rim 70 of the die 65 engageswith the recessed portion 80 of the core tube 75 to hold the die 65 andthe core tube 75 together.

The first end of the core tube insert 88 is inserted through the secondend of the core tube 75 in the aperture 81, and into thefigure-eight-shaped aperture 66 of the die 65. As so inserted, the endof the conical portion 89 abuts the step 84 of the core tube 75, theconical portion 89 of the core tube insert 88 contacts the conicalsurface 83 inside of the core tube 75, and the surface of the extension90 contacts the surface 82 of the core tube 75. Also, as so inserted,the tubes 91 extend into the figure-eight-shaped aperture 66 of the die65 so that the outer surfaces of the tubes 91 oppose respective circularsurfaces defining the figure-eight-shaped aperture 66.

The first end of the core guide 94 is joined with the second end of thecore tube 75 so that the conical portion 95 is inserted into theaperture 81 of the core tube 75 and meets with the conical surface 83thereof. The flat surface 96 of the core tube insert 88 thus abuts thesecond end of the core tube insert 88 to hold the core tube insert 88inside of the core tube 75 and the die 65. The aperture 97 of the coreguide 94 thus communicates with the apertures 92 of the core tube insert88 at the second end thereof. Also, the recessed portion 98 receives therim 85 of the core tube 75 to hold the core guide 94 and the core tube75 together.

The assembled die 65, core tube 75, core tube insert 88 and core guide94 are then inserted, with die 65 being inserted first, into the cavity11 defined in the crosshead 10 until the ledge 69 of the die 65 abutsthe step 62 of the crosshead 10. The threads 100 of the crosshead nut 99are then threaded to mating threads 13 of the crosshead 10, to hold thedie 65, the core tube 75, the core tube insert 88 and the core guide 94in position inside of the crosshead 10.

In preparation for making the zip-cord duplex optical fiber cable 1 ofthis invention, the buffer 5 is extruded on the optical fiber 4, whichis preferably coated, using well-known techniques, to make a bufferedoptical fiber. If the buffer 5 is to be relatively thick (e.g., 900 mmin diameter), a substance such as Modaflo™ is applied to the opticalfiber 4 to aid in stripping the buffer 5 when attaching a connector(s)for example, to the zip-cord duplex optical fiber 1. The application ofthe substance such as Modaflo™ can be applied to the outer surface ofthe optical fiber 4 by drawing the optical fiber 4 through a containerholding such substance. On the other hand, if the buffer 5 is relativelythin (e.g., 500 microns in diameter), the slick substance 6 (such astalc) is applied to the outer surface of the buffer 5 to allow it toslip relative to the yarn layer 7 to avoid breakage of the optical fiber4 which could occur, for example, if the buffer 5 is unable to sliprelative the yarn layer 7 during connection of a connector(s) attachedto the duplex optical fiber cable 1, to a connector receptacle(s). Theslick substance 6 can be applied by spraying or dusting the opticalfiber 6 with the substance 6 as the optical fiber 4 and its buffer 5 areadvanced in a linear direction. Alternatively, the optical fiber 4 andthe buffer 5 can be advanced through a container containing the slicksubstance 6 for the application of the substance 6 to the outer surfaceof the buffer 5.

The optical fiber 4 with the buffer 5 are then advanced through thecenter of an organizer (not shown) which can have a shape conforming toa ring. The ring has apertures radially arranged about the circumferencethereof, which receive respective strands to form the aramid yarn layer7. The yarn strands can be laid straight (i.e., parallel to the opticalfiber 4) by advancing the strands through respective holes in theorganizer and guiding the strands into contact with the buffer 5 to formthe yarn layer 7. Alternatively, the aramid yarn strands can behelically wrapped about the buffer 5 to form the yarn layer 7. As theoptical fiber 4 with its buffer 5 are advanced through the center of theorganizer, a motor (not shown) drives the organizer to rotate and thushelically wrap the yarn strands onto the outer surface of the buffer 5,to form the yarn layer 7. The yarn strands can be supplied fromrespective spools which unwind yarn strands as the optical fiber 4 andits buffer 5 are advanced through the center of the organizer. The aboveprocedure is repeated for a second optical fiber to be used in the pairof optical fibers 4 of the zip-cord duplex optical fiber cable 1.

The above procedures result in two separate optical fibers 4 withrespective buffers 5 and yarn layers 7.

The ends of the optical fibers 4 with respective buffers 5 and yarnlayers 7 are inserted into the aperture 102 of the crosshead nut 99 andalso inserted into respective sides, of the figure-eight-shaped aperture97. Importantly, as best seen in FIG. 8, the apertures 97 are tapered orfunnel-like in shape and as such allow for easy insertion of the ends ofthe optical fibers 4, buffers 5 and yarn layers 7 therein. Upon furtherinsertion, the ends of the optical fibers 4, buffers 5 and yarn layers7, are inserted into respective apertures 92 and through the ends of thetubes 91 of the core tube insert 88 and further through the aperture 12of the crosshead 10. At the left of the crosshead assembly 9 in FIG. 8,the ends of the optical fibers 4, buffers 5 and yarn layers 7, arecoupled to a motor (not shown) which draws them through the crossheadassembly 9.

As the optical fibers 4, respective buffers 5 and yarn layers 7, aredrawn through the crosshead assembly 9 with the motor, molten plasticmaterial such as nylon or PVC, is forced through the aperture 64 of thecrosshead 10 and into the core tube 75 through the notch 76. The flow ofmolten plastic material is split by passages 77 and uniformlydistributed about the outside of the conical portion 78 of the core tubeinsert 75 via recessed surfaces 79 and the inner surfaces of conicalsurface 73 of the die 65. The opposing conical surfaces 78, 73 of thecore tube 75 and the die 65, respectively, cause the split flows ofmolten material to converge as they flow toward the tip of the conicalportion 78, a feature of this invention which enhances the density, andtherefore uniformity, of the extruded sheath 8. The flow of moltenplastic material passes over the surfaces of the extension 90 and tubes91 and are shaped by the surfaces of the die 65 defining the outersurface of the sheath 8. As the sheath 8 cools after extrusion from thecrosshead assembly 9 of FIG. 8, the sheath 8 constricts to a degree andcontacts the yarn layers 7 to form a zip-cord duplex optical fiber cable1 as shown in FIG. 6. Preferably, the molten plastic material formingthe sheath 8 is PVC extruded at a temperature of 180° C. and cooled byimmersion in 40° C.-60° C. water.

FIG. 9 is a cross-section of a second embodiment of a duplex opticalfiber cable 1 of this invention. Essentially, the second embodiment ofthe duplex optical fiber cable 1 includes two simplex optical fibercables 1 as shown in FIG. 3 which are positioned side-by-side in asubstantially parallel relationship. Extruded to substantially surroundand contact the two simplex optical fiber cables 1, an oversheath 103 isformed. The oversheath 103 includes opposing, on the substantiallyparallel sides 104 with ends joined by respective semi-circular sides105 which are rounded to conform to respective outer surfaces of thesheath 8 of respective simplex optical fiber cables 1.

To attach connectors to respective ends of the two simplex opticalfibers 1 contained in the oversheath 103, the oversheath 103 can bestripped from an end of the duplex optical fiber cable 1 to free theends of the two simplex optical fiber cables 1. Connectors can then beattached to the ends of each simplex optical fiber cable 1.

Preferably, the duplex optical fiber cable 1 of FIG. 9 is incross-section about 1.6 mm to 2.1 mm in height and 2.76 mm to 4.2 mm inwidth, but preferably is in two standard sizes, one being 1.68 mm inheight and 2.76 mm in width and the other being 1.6 mm in height and 4.2mm in width. In the first standard size, the optical fibers 4 are about0.250 mm in diameter, the buffers 5 are about 0.13 mm in radialthickness, the yarn layers are about 0.22 mm in radial thickness and thesheaths 8 are about 0.18 mm in radial thickness. In the second standardsize, the optical fibers 4 are about 0.25 mm in diameter, the buffers 5are about 0.32 mm in radial thickness, the yarn layers are about 0.22 mmin radial thickness and the sheaths 8 are about 0.18 mm in radialthickness.

The die 9 used to make the second embodiment of the duplex optical fibercable 1 of this invention is substantially similar to that shown in FIG.7, except that the die 65 has an aperture 66 conforming in shape to theouter surface of the oversheath 103 shown in FIG. 9 rather than thefigure-eight-shaped configuration of FIG. 7, and the apertures 92 of thetubes 91 of the core tube insert 88 are sized to receive respectivesimplex optical fiber cables.

Advantageously, due to the application of the slick substance 6, thesimplex and duplex optical fiber cables 1 of this invention allow thebuffer 5 to slip relative to the yarn layer 7 when the ferrule of aconnector at the terminal end of the optical fiber cable 1 is connectedto a connector receptacle. This feature of this invention preventsfatigue or damage of the optical fiber(s) 4 in the optical fiber cable 1due to overbending which would otherwise occur with relatively thinbuffers 5 (e.g., 500 microns in diameter) in the absence of the slicksubstance 6. On the other hand, if the buffer 5 is relatively thick(e.g., 900 microns in diameter), a slippery substance such as Modaflo™is applied to the outside of the optical fiber(s) 4 so that the greaterhoop stress of the relatively thick buffer 5 will not impede thestripping of the buffers 5 to attach connectors, for example. Inaddition, the crosshead assembly 9 used to manufacture the simplex andduplex optical fiber cables 1 of this invention extrude a relativelyuniform sheath 8 or oversheath 103 which provides increased protectionfor the optical fiber(s) 4 relative to the nonuniform extrusions ofsheath material of previously used optical fiber cables. This advantageis derived from distributing the molten plastic material in split flowsusing passages 37, 38 of the core tube 36 of FIG. 4 or passages 77 ofthe core tube 75 of FIG. 7 to extrude molten plastic material uniformlyat various points around the outer surfaces of the yarn(s) 7 of thesimplex or duplex optical fiber cables 1 of this invention. In addition,the conical portion 51 and the funnel-like portion 32 (FIG. 4) or theconical portion 78 and the conical surface 73 (FIG. 7) cause the splitflows of plastic material to converge, thus increasing the uniformity ofthe extruded plastic sheath 8 or the oversheath 103. Moreover, thecrosshead assemblies 9 of this invention have funnel-shaped apertures56, 48 (see FIG. 5) or 97, 92 (see FIG. 8) when assembled which allowsthe ends of the optical fiber(s) 4, the buffer(s) 5 and yarn layer(s) 7,to be threaded relatively easily into the crosshead assembly 9 inpreparation for extruding the sheath 8 or the oversheath 103. Inaddition, the die 29, the core tube 36 and the core tube insert 46 ofFIG. 4 and the die 65, core tube 75 and the core tube insert 88 of FIG.7 are relatively easy to replace if they become fouled with extrudedplastic, for example, relative to previous crosshead dies which requiredthe replacement of the entire crosshead die rather than an individualcomponent such as the dies 29, 65, the core tubes 36, 75 or the coretube inserts 46, 88 of this invention. In the first and secondembodiments of the duplex optical fiber cable 1 of this invention shownin FIGS. 6 and 9, each optical fiber 4 and its buffer 5 are individuallywrapped with the yarn layer 7 as opposed to wrapping yarn aboutside-by-side buffered optical fibers, as done in a previous opticalfiber cable. This feature of the invention eliminates the need forbifurcation kits for connecting the end of a duplex optical fiber cableto a pair of connectors, which require components that greatly increasetie diameter of a duplex optical fiber cable using a bifurcation kit.The increased size of the optical fiber cable using a bifurcation kitgreatly increases the complication and congestion in racks or troughs ofcabinets, a problem which is overcome by this invention.

Although the invention has been described with specific illustrationsand embodiments, it will be clear to those of ordinary skill in the artthat various modifications can be made therein without departing fromthe scope and spirit of the invention as outlined in the followingclaims. For example, the buffer 5, the sheath 8, and the oversheath 103can be made of other materials than nylon or PVC as disclosed herein,such as halogen or non-halogen or plenum-rated plastic materials. Also,although the yarn 7 is preferably made of aramid fibers, other types ofstrength yarns can be used without departing from the scope of thisinvention. Moreover, the crosshead dies 9 of FIGS. 4 and 7 are shown byway of illustration of the principles of this invention only, andvarious modifications such as forming one or more parts of the crossheadassembly 9 together or even forming the dies and passageways of the coretubes or core tube inserts differently, can be done without departingfrom the scope of this invention, the important feature with respect todesigning the crosshead assembly 9 being that the molten material isdistributed at more than one point leading into the die forming theoutside of the sheath 8 or the oversheath 103, and that the flow ofmolten material converges to increase the uniformity of the sheath 8 orthe oversheath 103. All these modifications are intended to be includedwithin the scope of the invention as outlined in the following claims.

We claim:
 1. A method for making an optical fiber cable, the methodcomprising the steps of:a) receiving a flow of molten plastic materialwith a core tube; b) splitting the flow of molten plastic material inthe core tube; c) supplying the flows split in said step (b) toseparated locations defined at annularly-spaced intervals in the coretube; d) molding the split flows of molten plastic material from theseparated locations, between a surface of a die defining an aperturewith at least one circular hole less than 2.0 mm in diameter, and an endportion of a core tube insert having an end portion extending into theaperture of the die; and e) advancing at least one optical fiber havinga respective buffer and yarn layer, through an aperture defined in thecore tube insert, the core tube and the die so that the molding of saidstep (d) forms a sheath about the yarn layer, the buffer and the opticalfiber.
 2. A method as claimed in claim 1, further comprising the stepof:f) channeling the split flows supplied in said step (c) betweenrespective opposing conical surfaces of the core tube insert and the dieso that the flows converge and increase in density and uniformity,before performing said step (d).
 3. A method for making an optical fibercable as claimed in claim 1, further comprising the step of:applying aslick substance to an outer surface of the buffer.
 4. A method formaking an optical fiber cable as claimed in claim 3, wherein the slicksubstance includes talc.
 5. A method for making an optical fiber cableas claimed in claim 1, further comprising the step of:applying afriction-reducing substance to an outer surface of the optical fiber. 6.A method for making an optical fiber cable as claimed in claim 5,wherein the friction-reducing substance comprises PTFE and acetone.
 7. Amethod as claimed in claim 1, wherein the sheath is a first sheath, andfurther comprising the steps of:extruding molten plastic material toform a second sheath with a circular cross-section having an outersurface less than 2.0 mm about a second yarn layer surrounding and incontact with a second buffer of a second optical fiber; and extrudingmolten plastic material to form an oversheath surrounding and in contactwith the first and second sheaths, the oversheath having across-sectional height less than 2.0 mm and a cross-sectional width lessthan 4.0 mm.
 8. A method as claimed in claim 1, and wherein the sheathis a first sheath, and wherein the extruding of the first sheath in saidstep (a) is performed about a second yarn layer surrounding and incontact with a second buffer of a second optical fiber, so that thefirst sheath has two intersecting portions with respective circularouter surfaces less than 2.00 mm in diameter surrounding and in contactwith the respective first and second yarn layers, and having arelatively thin portion connecting the two intersecting portions, thesheath having a cross-section conforming to a figure-eight shape, thesheath having a cross-sectional height less than 2.25 mm and across-sectional width less than 4.60 mm.
 9. A method as claimed in claim1, wherein the flows of step (c) are defined as first and second primaryflows, and wherein each of the primary flows is divided into first andsecond secondary flows, each of the secondary flows being supplied toone of the separate locations.
 10. A method as claimed in claim 7,wherein the oversheath has a cross-section conforming to a figure-eightshape.
 11. A method as claimed in claim 9, wherein the separatelocations are annularly-spaced at ninety (90) degree intervals about acircumference of the core tube.